For conventional laser diode based systems, it is desirable to power laser diodes with an electrically efficient and mechanically compact power supply. For this purpose, a switch-mode power supply is often used. A switch-mode power supply is an electronic power supply unit including a switching regulator. The switch-mode power supply actively switches a transistor between full saturation and full cutoff at a high rate. A resulting rectangular waveform is then passed through a low-pass output filter, typically an inductor and capacitor, to achieve an approximated output voltage.
FIG. 1 illustrates the output filter stage of a conventional switch-mode power supply. The output filter includes an LC circuit, including an inductor L and a capacitor C. The output filter smooths the inherently pulsed output of the power supply. A rectangular waveform is passed through the output filter to effectively generate an output voltage across the capacitor C. The switch-mode power supply is configured to behave as a low bandwidth current source, which is required for the proper operation of a series of coupled laser diodes, but becomes a high frequency voltage source due to the presence of the output filter.
FIG. 2 illustrates a circuit 2 including a series of laser diodes coupled to the switch-mode power supply including the output filter of FIG. 1. The laser diodes D1, D2, D3, D4, D5 and the resistor R are coupled in series. The series of laser diodes and the resistor R are coupled across the capacitor C of the output filter. Although the series of laser diodes is shown in FIG. 2 as including five laser diodes, the series of laser diodes can include more or less than five laser diodes.
When such a switch-mode power supply is used to provide power to a set of laser diodes connected in series, problems arise when one of the laser diodes fails, effectively becoming electrically shorted. In such a situation, the forward bias voltage of the series of laser diodes instantaneously decreases. Within the switch-mode power supply, the instantaneous current of the capacitor C is proportional to the product of time rate voltage change and the capacitance of the capacitor C. When a laser diode fails, effectively electrically shorting, the result is a large surge of current from the capacitor C through the series of laser diodes. The size of the current, given by Ohm's Law I=V/R, can be catastrophic owing to the small size of the equivalent series resistance R and the extreme sensitivity of the laser diodes to damage by momentarily excessive current.
Another problem resulting from electrically connecting a number of laser diodes is intermittent connection. Intermittent connection occurs when a contact within the laser diode series is momentarily open, at which point the power supply drives up the voltage across the capacitor in the output filter in an attempt to compensate for the current short fall. When the intermittent contact is reestablished, the extra voltage across the capacitor can cause a damaging current spike across the series of laser diodes. Since laser diodes are expensive and fragile devices, their protection is a critical consideration in any system design.
Conventional current limiting circuits cannot be used to solve this problem due to their finite bandwidth. The finite bandwidth precludes the current limiting circuits from protecting the laser diodes against rapidly changing current.